#3 Populate the world through Incest (Adam, Eve, Caine and Abel the only humans on Earth - you do the math)
A more full account of the lives of Adam an Eve and their children can be found in the Book of Moses:
Moses 5:1-18
1 And it came to pass that after I, the Lord God, had driven them out, that Adam began to till the earth, and to have dominion over all the beasts of the field, and to eat his bread by the sweat of his brow, as I the Lord had commanded him. And Eve, also, his wife, did labor with him.
2 And Adam knew his wife, and she bare unto him sons and daughters, and they began to multiply and to replenish the earth.
3 And from that time forth, the sons and daughters of Adam began to divide two and two in the land, and to till the land, and to tend flocks, and they also begat sons and daughters.
4 And Adam and Eve, his wife, called upon the name of the Lord, and they heard the voice of the Lord from the way toward the Garden of Eden, speaking unto them, and they saw him not; for they were shut out from his presence.
5 And he gave unto them commandments, that they should worship the Lord their God, and should offer the firstlings of their flocks, for an offering unto the Lord. And Adam was obedient unto the commandments of the Lord.
6 And after many days an angel of the Lord appeared unto Adam, saying: Why dost thou offer sacrifices unto the Lord? And Adam said unto him: I know not, save the Lord commanded me.
7 And then the angel spake, saying: This thing is a similitude of the sacrifice of the Only Begotten of the Father, which is full of grace and truth.
8 Wherefore, thou shalt do all that thou doest in the nameof the Son, and thou shalt repent and call upon God in the name of the Son forevermore.
9 And in that day the Holy Ghost fell upon Adam, which beareth record of the Father and the Son, saying: I am the Only Begotten of the Father from the beginning, henceforth and forever, that as thou hast fallen thou mayest be redeemed, and all mankind, even as many as will.
10 And in that day Adam blessed God and was filled, and began to prophesy concerning all the families of the earth, saying: Blessed be the name of God, for because of my transgression my eyes are opened, and in this life I shall have joy, and again in the flesh I shall see God.
11 And Eve, his wife, heard all these things and was glad, saying: Were it not for our transgression we never should have had seed, and never should have known good and evil, and the joy of our redemption, and the eternal life which God giveth unto all the obedient.
12 And Adam and Eve blessed the name of God, and they made all things known unto their sons and their daughters.
13 And Satan came among them, saying: I am also a son of God; and he commanded them, saying: Believe it not; and they believed it not, and they loved Satan more than God. And men began from that time forth to be carnal, sensual, and devilish.
14 And the Lord God called upon men by the Holy Ghost everywhere and commanded them that they should repent;
15 And as many as believed in the Son, and repented of their sins, should be saved; and as many as believed not and repented not, should be damned; and the words went forth out of the mouth of God in a firm decree; wherefore they must be fulfilled.
16 And Adam and Eve, his wife, ceased not to call upon God.
And Adam knew Eve his wife, and she conceived and bare Cain, and said: I have gotten a man from the Lord; wherefore he may not reject his words. But behold, Cain hearkened not, saying: Who is the Lord that I should know him?
17
And she again conceived and bare his brother Abel. And Abel hearkened unto the voice of the Lord. And Abel was a keeper of sheep, but Cain was a tiller of the ground.
18 And Cain loved Satan more than God. And Satan commanded him, saying: Make an offering unto the Lord.
Marrying close relatives was not an issue in the beginning of the world and there was no law against it. In fact the sons and daughters of Adam and Eve were commanded to multiply and replenish the earth. Over a long period of time genetic abnormalities began to show up and for this reason incestual relations became something to avoid. The Lord in the Law of Moses began to lay down laws against incestual relations many years after Adam and Eve:
Leviticus 18
Leviticus 20
Deuteronomy 27